When converting my doc files I haven't had problems with first, saving as html with the "Save As.." "Web page, filtered" option on my Mac, adding this to my calibre library then converting this to mobi.

However I recently upgraded to Word 2011 for Mac which doesn't have this option -- only "Save As Web Page". Since then, almost all my docx-saved-as-html files hopelessly add as .htm instead of .zip to calibre which results in conversion error. I think it has to do with the long string of css tags at the beginning of the html code.

The best success you'll have is NOT t allow Word to touch any file you want to have as an eBook. Word makes one hell of a mess to your HTML/CSS. It takes you more time to fix the mess then it does to read the content.